The prior art includes nonreturn or unidirectional valves of the general type with which the present invention is concerned. One example of such a valve is found in German Auslegeschriften No. 1,475,998, which has an insertion member installed axially in a housing, the member being connected with crosspieces to a ring forming a sealing surface on the front, or upstream, side, and is fastened in the housing by the ring. The valve seal, with the external diameter of the rigid, ring-shaped part is inserted directly in a cylindrical guide of the housing so that it can move axially. During the closing movement it is supported on a cylindrical part of the insertion member with the interior diameter of the flexible part. Perfect guidance of the seal during the axial movement is not guaranteed by this arrangement which results in tilting of the seal and, thus, in operational breakdowns. Since the sealing takes place at the largest diameter of the joint, only small specific surface pressures occur at the sealing surface. For that reason, a sufficient sealing effect is not obtained when there are small differences in pressure in the line. Furthermore, large differences in pressure are necessary for opening the flexible seal lip because of its placement at the inner part of the seal. Because of these disadvantages, the assembly does not function reliably when it is installed in the horizontal position and under conditions of small differential pressure or small flow quantities.